The present invention relates to the techniques which make it possible to modify the optical properties of a transparent polymer, more particularly when it is sought to modulate the refractive index of a semifinished article of transparent organic material which has previously been polymerized and shaped, without substantially modifying its geometry or its state of surface.
Under these conditions, a preferred application of the invention relates to the manufacture of artificial optical lenses for correcting eyesight such as contact lenses and ocular implants in view of the fact that, in these examples, it is of great importance to preserve a predetermined geometrical shape as well as qualities of surface which ensure, on both faces of the lens, biological compatibility with the ocular medium and absence of irritation of the eye.
The distinctive feature of lenses of this type lies on the one hand in the fact that they have small dimensions and are intended to be employed in full aperture in contradistinction to spectacle lenses, and on the other hand in the fact that they must have a predetermined geometrical shape which will be adapted to the morphology of the wearer's eye in the case of contact lenses, for example, or which will provide basic accommodation power in the case of an implant for the replacement of the crystalline lens of the eye.
In this type of lens, the refractive index modulation contemplated by the invention may be intended in particular to produce a diffraction grating which endows a contact lens with bifocality or an implant for far-sighted persons. In the case of a contact lens, for example, this makes it possible to correct the user's near vision with a power which is determined by this element while correction of distant vision is achieved independently by the geometry of the lens.
It is at present known to construct contact lenses with diffraction elements by modulating the relief of the outer or inner face of the lens. This modulation can be obtained by a machining technique, by etching the lens material at the desired pitch in order to form the diffraction grating. This technique not only calls for the use of equipment involving high cost and delicate maintenance but has the additional disadvantage of being reserved for contact lenses of the rigid type. In regard to molding of the lens directly with the desired relief, this operation calls for preliminary construction of a mold which is just as costly and difficult to machine and is used for a short period of time.
In one case as in the other, the lenses obtained prove unsatisfactory. When the lens has been worn for a certain time, fouling of the diffraction element is observed. This phenomenon is particularly marked when the diffraction element is formed on the inner face of the lens. Moreover, in commercially available lenses of this type and for reasons of adaptation, the curvature of the inner face of the lens has been increased in order to minimize the contact surface between the etched face of the lens and the cornea, thereby increasing the thickness of the tear film and thus producing an additional lens effect having optical characteristics which are not fixed in time by reason of the instability of this tear film.
Another known method for forming diffraction-element contact lenses consists, as described in European patent No. 0,064,812, in applying on the lens a layer of gelatin which is light-sensitized by bichromate treatment and then exposed so as to form a diffraction grating thereon. It is proposed by way of alternative that the gelatin layer can be included within the lens as an intermediate layer in order to avoid any direct contact with the eye since bichromate is known as an irritant compound. It hardly seems possible, however, to adopt this solution in industrial practice.
In point of fact, when it is located on the outer face of the lens, the gelatin layer gives rise to problems relating to insufficient adhesion to the substrate which is constituted by the material of the lens itself. In consequence, said layer is liable to be damaged by the movements of the eyelid and also to be subjected to variations in its degree of hydration by the ocular medium, and therefore to variations in swelling which result in instability of the optical properties of the diffraction element.
It is also known that there exist other methods for forming diffraction gratings on rigid supports by modulating in thickness and refractive index transparent films applied on said supports, this being achieved by producing local photopolymerization of a monomer which is incorporated with a prepolymer. Methods of this type have been proposed for forming waveguide films or holograms.
Since flat thin films are involved in contrast to the present invention, the known methods entail the need to deposit on a rigid support a liquid composition including the basic polymer in solution with the photopolymerizable monomer as well as a photoinitiator which is capable of causing polymerization of said monomer under the action of radiation. The deposit is then hardened by exposing it to a radiation source which is modulated in intensity, in power or in exposure time, or through a mask of suitable shape, whereupon the non-hardened constituents derived in particular from non-irradiated zones of the film are removed. As a result of the deficient stability in time of the refractive index modulations thus obtained, it has sometimes been recommended in addition to carry out a chemical reaction for fixing the monomer on the polymer in order to prevent excessively easy diffusion from destroying the index modulations. Supports of porous silica glass have also been employed for the same purpose.
When it is sought to apply these techniques to the field of optical lenses, it is naturally imperative to ensure that the index modulations achieved remain stable in time and are insensitive to the ocular medium. Moreover, another problem which does not exist in the case of films of the prior art arises from the need to conform to a predetermined lens geometry.
In point of fact, the techniques of screen printing by photosensitization which are applied in the prior art can in no event be directly transposed to refractive index modulation of a polymer to be employed as an artificial optical lens in the eye since they did not need to satisfy the same requirements of geometrical stability, of quality and of biocompatibility of their entire surface.
And when some investigators such as the authors of U.S. Pat. No. 4,778,256 have sought to treat objects and not films on a substrate, the variation involved was in depth and not parallel to the surface as is required by the lenses contemplated by the present invention. Furthermore, the objects treated were in this case also of porous silica glass and not of organic polymer.